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Interpretive Summary: Previous evidence has suggested that growth hormone (GH) plays a role in mediating ovarian function in the female. For example, in women, concentrations of GH have been shown to be elevated around the time of ovulation. Additionally, GH has been used in the human, as well as cattle, to increase ovulation rate when used in combination with reproductive releasing hormones. However, there have not been conclusive studies that directly relate circulating GH concentrations to ovulation. Thus, this study examined 1) whether naturally circulating concentrations of GH may be associated with the growth of follicles (the structure which contains growing/mature eggs) during the reproductive cycle of the cow; and 2) whether the release of GH may change in relation to follicular development and stage of the reproductive cycle. Results from this study indicate that while daily concentrations of GH do not differ during the bovine reproductive cycle, there are strong correlations among GH, steroids produced by the ovary, and the number of large follicles that develop. These results indicate that GH may work in concert with other hormones throughout the reproductive cycle to support development of follicles. This information enhances our understanding of how GH, in conjunction with other hormones, may be effectively used to treat ovarian disorders which are associated with reduced fertility.

Technical Abstract: Our objective was to determine (1) whether circulating concentrations of growth hormone (GH) may be associated with the growth of follicles during the estrous cycle in the cow; and (2) to examine whether GH secretory characteristics (i.e., number of GH peaks, peak height and amplitude) may change in relation to follicular development and stage of the estrous cycle. Blood was collected from 9 Brahman cows for d 1 following natural estrus (d 0) through the mid-luteal phase. At the appearance of the second 8 mm follicle following estrus (determined by transrectal ultrasonography), PGF2alpha was administered. Intensive blood sampling was conducted on d 3 of the estrous cycle, d of the first observed 8 mm follicle and 48 h after PGF2alpha injection; at which time cows were fitted with indwelling jugular catheters and blood collected at 10-min intervals for 6 h. Daily plasma samples were analyzed for progesterone (P4) and estradiol (E3), while samples collected daily as well as during intensive sampling periods were analyzed for GH by RIA. Plasma GH did not differ (P>.10) among intensive sampling periods. However, the amplitude of GH peaks tended (P<.09) to be greater at 48 h after PGF2alpha administration compared to d 3 of the estrous cycle or d of the first 8 mm follicle. Numbers of small follicles (5.0 mm) decreased (P<.05) during the first 12 d of the estrous cycle, while numbers of large follicles (8.0 mm) increased (P<.05) over time. Plasma GH on d 4 through 8 were positively correlated (r=0.73 to 0.87; (P<.02) with numbers of large follicles on d 7 and 8. In summary while mean daily GH did not differ during the estrous cycle, correlations among GH, ovarian steroids and numbers of large follicles support a role for GH in mediating follicular population dynamics in the bovine.